Cellulose nitrate



from about 0.25 to 0.5.

CELLULOSE NITRATE Arthur W. Sloan, Washington, D.C., and David J. Mann,

Wharton, N.J., assignors to Atlantic Research Corporation, Alexandria,Va., a corporation of Virginia No Drawing. Application August 13, 1954SerialNo. 449,778

"2 Claims. (Cl. 260-223 This invention relates to a new and improvedcellulose mtrate product in the form of small, spherical particles ofhigh density.

Processes for preparing ball powder from high nitrogen content cellulosenitrate (12.4-13.4% nitrogen) are described in Olsen et al.,' U.S.2,027,114 and Shaefer, US. 2,160,626. In the former patent, a colloid,such as starch, is used to obtain sphere formation and the products arehighly porous, with specific gravities ranging In the latter patent, asolute such as sodium sulfate, is added to the aqueous phase of theemulsion to increase density of the cellulose nitrate spheres. It isstated that specific gravities as high as 0.8 to 1.0 may be obtained.However, in view of actual specific gravity of high nitrogen contentcellulose nitrate of 1.58 to 1.66 depending upon degree of nitration, itis evident that a product having a specific gravity of 0.8 to 1.0possesses a high degree of porosity. For some uses, considerably denserspherical particles are required. An additional disadvantage of theseprocesses stems from the fact that the smallest particle produced isabout 6 mils or 150 microns and these are obtainable only as a screenedfraction of a batch containing a large proportion of considerably largerspheres. Thus these processes cannot meet a requirement for cellulosenitrate spheres smaller than 6 mils and even the smaller sizes producedby these methods constitute only a portion of a given production.

The object of this invention is to provide small, substantiallynon-porous, spherical particles of cellulose nitrate of high nitrogencontent, namely cellulose nitrate containing more than 12% nitrogen.

Processes for making such small, non-porous spherical particles ofcellulose nitrate are described and claimed 1n Sloan and Mann patentapplication Serial Number 449,777, filed concurrently herewith, nowPatent No. 2,891,055, granted June 19, 1959. i As disclosed in saidco-pending Sloan and Mann application, particles having the desiredcharacteristics of size, sphericity and substantial non-porosity may beobtained when the cellulose nitrate in specified concentration isdissolved in an organic solvent characterized by certain essentialproperties and the resulting lacquer is dispersed with vigorousagitation in specified amounts of water in the presence of certaincolloids together with water-soluble salts or water-soluble polyhydroxycompounds to form an oil-in-water type emulsion. In some cases it isalso desirable to introduce a polar-type, surfaceactive emulsifyingagent.

The cellulose nitrate solvent may be a single solvent which ismoderately soluble in water, a mixture of solvents, the components ofwhich are moderately soluble in water, or a mixture of solvents, atleast one componentof which is moderately soluble in water and anothercomponent of which is infinitely soluble in water. By moderately solubleis meant a solvent which is soluble in water at least to the extent ofparts per 100 parts of water at 20 C. but is not infinitely soluble. Byin- F A 2,931,801 Patented Apr. 5, 1960 finitely" soluble is meant asolvent which is soluble in water in all proportions. Maximum solubilityin water of the cellulose nitrate solvent should be about parts perparts of water and preferably about 40 to 50 parts. Thus, if a mixedsolvent is employed containing an infinitely soluble component, theamount of infinitely soluble component must be adjusted to maintaintotal solubility in water of the mixed solvent within the de siredrange.

Examples of moderately water-soluble organic solvents include methylacetate, ethyl acetate, methyl formate, ethyl formate, methyl ethylketone, diethyl ether and nitromethane. The moderately water-solublesolvents preferably range in water-solubility from about 5 to 30 partsper 100 parts of water at 20 C.

Infinitely water-soluble co-solvents include the lower aliphaticalcohols such as methyl, ethyl and propyl alcohols, acetone, dioxan-1,4.

Where co-solvent mixtures are used, it is not essential that each of thesolvent components be a good solvent for the cellulose nitrates so longas they possess good solvent properties when in admixture. Diethyl etherand the lower aliphatic alcohols, for example, though poor solvents forcellulose nitrate per se, possess excellent cosolvent properties. It isdesirable that the infinitely soluble component of a mixed cellulosenitrate solvent possess the ability to reduce viscosity of the lacquer.Methanol, ethanol and acetone are particularly effective in this regard.

We have found that particles having the desired characteristics cannotbe obtained with a cellulose nitrate solvent which is substantiallyinsoluble in water, namely soluble in water to the extent of less thanabout 5 parts per 100, or which is excessively soluble. The watersolubility must be sufficient so that when the lacquer particles aredispersed in water, the solvent at and adjacent to the surface dissolveswith sufiicient rapidity in the water to permit some hardening orsetting of the particle surface and thus eliminates or markedly reducesthe surface tackiness to the point where such surface tackiness will notcause agglomeration. 0n the other hand, elution of the solvent shouldnot be so rapid that the surface sets into a hard, non-plastic conditionbefore surface tension forces can efiectively shape the particles intothe desired spherical form. Premature excessive hardening of theparticle surface also causes porosity since the non-plastic surface doesnot permit adequate shrinkage to compensate for removal of solvent fromthe interior of the particle. With a cellulose nitrate solvent ofexcessive water-solubility, porosity may also be caused by migration ofwater into the particle because of the mutual solubility of water and ahighly water-soluble solvent. As aforementioned, the water-solubility ofthe cellulose nitrate solvent, whether single or mixed, should be withina range of about 5 to 80 parts per 100 parts of water at 20 C. andpreferably about 5 to 40 or 50.

We have found that to obtain spheres of the desired small size,concentration of the high nitrogen content cellulose nitrate in thelacquer should not exceed about 10% by weight. Optimum concentration isabout 5 to 7.5%. Lacquers containing more than about 10% of the highnitrogen cellulose nitrate are highly viscous and are comminuted withdifiiculty to produce particles of generally excessive size. There is nocritical lower limit of cellulose nitrate concentration in the lacquerother than that dictated by economic expediency. Good remethyl lactate,ethyl lactate, and.

g-ree the cellulose nitrate concentration in the lacquer. Viscosity ofthe lacquer varies with different solvents and the concentration ofcellulose nitrate may be varied accordingly.

Solvent systems which we have found to be particularly satisfactoryinclude methyl acetate, methyl acetate/ methanol, ethyl acetate, ethylacetate/ethanol, ethyl acetate/ acetone, ethyl acetate/diethyl ether,methyl ethyl ketone/ acetone, methyl ethyl ketone/ methanol, andnitromethane.

Where a mixed cellulose nitrate solvent as, for example, one containingan infinitely soluble component, is employed, the ratio of componentsfor optimum performance can readily be determined by routineexperimentation. In most cases a ratio of moderately soluble componentto infinitely soluble component of 80:20 to 85:15 by volume isparticularly good. However, this may be varied so long as totalsolubility of the mixed solvent does not become excessive.

Choice of solvent is also influenced to some extent by the particularcolloiding agent employed. The protective colloid must not beexcessively soluble in the cellulose nitrate solvent since, otherwise,the system would tend to form a water-in-oil type emulsion. Thissituation can be handled by employing a colloid which is not highlysoluble in the specific solvent or by avoiding the use of solvents whichtend to dissolve the particular colloid out of the water phase.

The cellulose nitrate lacquer is mixed with water in the presence of asuitable colloiding agent and a watersoluble salt or polyhydroxycompound with vigorous agitation to form a dispersion of small sphericallacquer particles with the water as the continuous phase. The colloidand salt are preferably introduced into the Water prior to mixing. Thedispersion and agitation may be accomplished by any suitable means as,for example, in a homogenizer or other suitable agitating device. Thedegree of agitation is one of the factors influencing particle size. Ingeneral, the more vigorous the agitation, the smaller are the particles.

The amount of water employed in dispersing'the cellulose nitrate lacqueris an important factor in determining particle size. Particle size is afunction of the lacquer/water ratio inasmuch as increasing the amount ofwater increases particle size. Thus the quantity of water used inemulsification is determined in some measure by the particle sizedesired. The minimum amount of water is governed by the lacquer/waterratio required to permit formation of the requisite oil-in-water typeemulsion. If excessive water is employed, the particles become oversizeand may also become irregular and porous, apparently because the organicsolvent dissolves in the water phase with excessive rapidity so that thesurface of the particles hardens into a non-plastic condition beforeadequate comminution or contraction into spheres. In general, themaximum amount of water which gives satisfactory results in terms ofdesired particle characteristics is about 100% by volume based on thevolume of organic solvent and preferably about 50 to 75%.

We have found that particle characteristics such as size and density aregreatly influenced by the particular colloid employed. Non-porousparticles within the desired size range can be obtained with thefollowing colloiding agents; methyl cellulose, casein, thecondensationproduct of polyvinyl methyl ether and maleic anhydride,gelatin, agar-agar, polysodium acrylate, polyvinyl alcohol, andalginates.

The colloiding agent must be preferentially soluble in water rather thanin the organic solvent to avoid formation of a water-in-oil system.Although the aforementioned colloids are normally hydrophilic, certainones may be excessively soluble in a particular organic solvent. Methylcellulose, for example, cannot be employed with ethyl acetate because ofits high solubility in this solvent, although it functions well withother solventssuch as sea-em r e 4 7 methyl ethyl ketone/ acetone. It isessential, therefore, to employ organic solvent-colloid systems of lowmutual solubility relative to the solubility of the colloid in thewater. The nitromethane-polyvinyl methyl ether-maleic anhydride system,for example, is excellent because of the insolubility of the colloid inthis solvent.

The colloiding agent forms a colloidal solution with the water, thusincreasing the viscosity of the water phase, and also surrounds thelacquer particles with a protective coating. These factors stabilize theemulsion by reducing tendency of the particles to coalesce, particularlyduring stripping of the solvent. This is important since the particlesmust be maintained in a state of dispersion until surface tackiness hasbeen substantially eliminated and hardening has progressed to the pointwhere the particles will no longer agglomerate.

The amount of colloiding agent employed is determined by severalfactors. Primarily, it should be sufficient to produce substantialincrease in the vicsosity of the water phase and to coat the particles,thus providing adequate stabilization of the emulsion during processing.Generally speaking, the higher the molecular weight of the colloid, theless will be required. The colloid is also a factor in controllingparticle size since increasing concen tration tends to reduce particlesize. There is no critical upper limit to the amount of colloid employedother than practical considerations such as desired particle size andease of washing. The more colloid present, the more washing is requiredfor its removal.

Colloid-organic solvent systems which we have found particularlysuitable for our purpose include casein plus ethyl acetate, methylcellulose plus methyl acetate, methyl ethyl ketone/ acetone or methylethyl ketone/methanol, polyvinyl methyl ether-maleic anhydride plusnitromethane or ethyl acetate, alginates plus ethyl acetate, and gelatinplus ethyl acetate. The methyl cellulose is preferably of the highermolecular weight variety, namely one having a minimum centipoise valueof about 400 and preferably about 1500 to 4000 or higher.

It is essential that a suitable water-soluble salt or an organicpolyhydroxy compound be included in the water phase of the emulsionsystem; it is apparently necessary for the proper functioning of theprotective colloid. Without the addition of such compounds, theparticles formed are non-spherical and oversize. The salt or polyhydroxycompound prevents the inversion of phase which frequently occurs whenthe emulsion is heated to distillation temperature to remove the organicsolvent. It may be, also, that these additives exert a stabilizingeffect by causing hydration of the colloid and thus preventing unduepenetration of the colloid into the lacquer particle, and, in the caseof the salts, by inducing a charge on the surface of thecolloid-enveloped particle which tends to prevent agglomeration.

Any suitable, water-soluble salt which is compatible with the colloidingagent, namely one which will not cause it to precipitate, may beemployed as, for example, the metal or ammonium halides, sulfates,acetates and the like. Choice of the particular salt employed is, insome measure, determined by the particular colloid used in the emulsionsystem. All of the colloiding agents except methyl cellulose functionWell with salts of any valence, preferably monovalent or divalent salts,as, for example, sodium chloride or sodium sulfate. Methyl cellulose,however, tolerates only monovalent salts such as the alkali metal andammonium halides. By monovalent salt is meant one in which both thecation and anion are monovalent. By divalent salt is meant one in whichat least one ion is divalent. In general, inorganic salts havingmonovalent cations are preferred, the alkali metal salts being mostdesirable.

The salt must be employed in an amount less than that which will causeprecipitation of the colloid or a salting out effect. :In general, thelower the molecular weightof the.colloid,thegreater the amount of saltwhich it will tolerate. Degree of salt toleration also varies with theparticular salt.

We have found that below certain minimum salt concentrations the salt isnot effective since the resulting particles tend to be oversize andnon-spherical. This minimum amount varies with the particular salt. Ingeneral, the minimum concentration for the monovalent salts is about6.5% based on the weight of the cellulose nitrate and for divalentsalts, such as sodium sulfate, about 13%.

Addiitves other than salts which we have found to perform satisfactorilyare water-soluble polyhydroxy compounds such as sugars, glycerin andglycols such as ethylene glycol. In general, a minimum of about 6.5% onthe cellulose nitrate is desirable for satisfactory performance.

In some cases, it may be desirable, though not essential, to include inthe system a polar-type, surface-active emulsifying agent which producesa marked lowering of interfacial tension and which, unlike the highermolecular weight colloiding agent, does not produce any appreciableincrease in the viscosity of the water phase. The surface-activeemulsifying agent is advantageous inasmuch as in some instances it makespossible a reduction in the amount of colloid required for optimumperformance and thus facilitates subsequent removal of the colloid fromthe particles. It also reduces the amount of energy and time required todisperse the lacquer in the water and to comminute it to the desiredparticle size.

Any suitable surface-active emulsifying agent may be employed which iscompatible with the other 'compo nents of the system and which issusbtantially soluble in water, namely possesses a sufliciently highhydrophilelipophile balance to prevent its being drawn into the organicsolvent and converting the emulsion into a water-in-oil system, may beemployed, such as alkyl sulfates or sulfonates, alkylaryl sulfonates,alkali metal soaps, alkali metal and ammonium salts of perfiuoro acids,

alkali metal salts of 'sulfosuccinic acids, sulfonated oils' includingsulfonated vegetable oils and sulfonated hydrocarbon oils, polyglycolsand the like.

The moderately bon oils as, for example, sulfonated petroleum fractions,

alkali metal fatty acid soaps, polyglycols such as polyethylene glycoland polypropylene glycol, and the like, are especially satisfactory. Thepolyglycols, in addition to their dispersing action, also may beadvantageous as coupling agents, namely as agents which increasesolubility of the colloid 'or the organic solvent in water.

The amount of surface-active emulsifier is not critical but should besufficient to promote the desired rapid emulsification and comminutionof the particles. As little as about 0.01 to 0.25% based on the waterphase may be adequate. The amount may be increased to as much as 5 to10% in some cases. Concentrations of emulsifier in the range of about0.01 to 2% are generally satisfactory. The emulsifier may be added tothe water or to the lacquer phase.

After emulsification is completed, the organic solvent is removed fromthe dispersed particles by distillation or by elut'ion. In either case,the emulsion should be main tained in a state of continuous vigorousagitation.

Distillation is accomplished by heating the emulsion to or near theboiling point of the organic solvent. If the distillation is conductedat atmospheric pressure, it is desirable that the solvent or the leastvolatile component of a mixed solvent has a boiling point below 100 C.to maintain stability both of the cellulose nitrate and of the emulsion.In many cases, it may be desirable to distill under reduced pressures,particularly if the boiling point is above 100 C.

Another effective method for removing the organic solvent is by dilutingthe emulsion with water in amount suflicient substantially completely todissolve the solvent out, of the cellulose nitrate particles. Since itis desirable to maintain the effective salt or polyhydroxy compoundconcentration throughout the disperse phase of the particles, it ispreferable to include these solutes in the elution water prior todilution of the emulsion. The total amount of water should be in excessof the theoretical amount required for solution of the organic solvent,preferably in substantial excess.

After removal of the organic solvent, the cellulose nitrate particlesare separated from the water, washed with water and dried. Removal ofthe colloid may re quire several washings. In general, the smaller theparticles, the more water washings are required.

The cellulose nitrate particles prepared according to our process arespherical and may be obtained in sizes as small as 1 micron or less indiameter. The particle size generally ranges up to about 10 or 30microns.

This is the preferred size range. Depending upon the specific componentsand concentrations employed in the process, particle size may range upto about microns. In general, products in which the average particlediameter is above about 50 microns are marginal. In other words, it ispreferable that the size distribution of the particles by number be suchthat the maximum average size is about 50 microns. Since all of theparticles in a given production batch are obtainable in the desiredsmall-size range, there is no necessity for fractional screening orreworking of excessively large particles.

Density of the cellulose nitrate spheres is high. Average density of thespheres is about 1.49 to 1.54 with a minimum of about 1.4 as comparedwith published cellulose nitrate densities of 1.58 to 1.66.

The small, dense, spherical particles of cellulose nitrate .are highlysuitable for use in the manufacture of smokeless powder charges andpropellants in any desired and conventional manner. They are especiallyuseful, however, in that they can be suspended in a non-volatileplasticizer to form homogeneous, stable fluid slurries' which may bepoured as coatings or films without requiring the addition of water or avolatile solvent and may be molded into objects of any desired shape andsize without the application of high temperatures and pressures.

'Nitroglycerin is readily incorporated with the cellulose nitrateparticles by introducing it into the lacquer. It

"may be added with the organic solvent prior to solution of thecellulose nitrate or it can be added to the cellulose 'nitrate-soulventsolution. Stabilizing agents such as diphenyl amine, lecithin, ethylcentralite and the like can similarly be added.

EXAMPLE I 20 grams of cellulose nitrate (12.6% N) were dissolved in 200ml. ethyl acetate to form a clear lacquer. 8 grams casein, 8 grams Na SOand 0.3 grams ethyl centralite were dissolved in ml. water. The aqueoussolution was added to the cellulose nitrate lacquer in a homogenizer.The emulsion was agitated vigorously for about 5 minutes and then washeated to about 77 C. to distil off the ethyl acetate from the dispersedspherical particles. Vigorous agitation was maintained duringdistillation. The product was washed with water 3 times, filtered,washed again with water and dried. The product was in the form ofspherical particles ranging in size from 2 to 20 microns with an averagesize of 12 microns and having a density of 1.54.

EXAMPLE II 10 grams of cellulose nitrate (12.6% N) were dissolved in 200ml. ethyl acetate to form a clear lacquer. 8 grams gelatin (Knox) and 14grams Na SO were dissolved in 150 ml. of Water. The aqueous solution wasadded to the cellulose nitrate lacquer in a homogenizer. The emulsionwas agitated vigorously for about 5 minutes. The organic solvent wasdissolved out of the spherical particles by flooding the emulsion withabout 4 volumesof water containing about 2% of the salt under continuedagitation. The product was filtered, washed with water about 3 times andthen dried.. The spherical particles ranged in size from 1 to 10 micronsand had a density of about 1.49.

Other examples illustrating our invention are summarized in Table I. Theparticles in all cases were spherical.

characterized by their ability .to be dispersed in a plasticizer to formstable, fluid suspensions.

2. Cellulose nitrate, containing more than about 12% nitrogen, in theform of substantially spherical, nonporous particles having a maximumdiameter of about 30 microns, and a minimum density of 1.4, saidparticles being characterized by their ability to be dispersed in aplasticizer to form stable, fluid suspensions.

Iable I Cellulose nitrate Solvent, ml. Colloid, grams Other additivesSolute, grams Wat er, Size, [A

III. Percent g. N i 20 Ethyl acetate, 200 Casein, 12 Ethyl centralit e,0.3 g.---. M11530, 8. 150 2-40, av. 20. 20 do Oasein,8 Ethyl ccntralite,0.3 g.; NaiSOi, 150 235,av.15. j Cilyiieryl monorieinolea e, g. 20 do dEthyl centralite, 0.3 g.; NaiSQi,8..-... 150 2-40, av. 20.

Polyethyleneglycol, 6 g. 20 do do Ethyl centralite, 0.3 g. Sugar, 8 15010-40, av. 20. 20 do do Ethylene glycol, 8-.- 150 2-30, av. 15. 20 do doGlycerin, 8 150 2-15. 20 do Casein, 12; Polyvinyl do N3zSO4, 8 150 2-30,av. 15,

met y ether-maleic anhydride, 1. 12.6 20 Ethyl'lzahcetate, 180; Etha-Casein, 8 d M11804, 8.......... 150 2-30, av. 15.

no 12.6 2O Ethyl acetate, 170; Acedo do Na1SO4,8- 150 10-35, av. 18

tone, 30. and 2-5. 12.6 20 Etlzlyl ae3ebtate, 170; Ethyl do NBiSOi, 8150 2-20, av. 15.

e er, 12.6 20 Ethyl acetate, 20 do do NazSOi, 4 150 10-40, av. 20. 12.620 Ethlyl acetate, 170; Ethyl do do Hexalene glycol, 8.-. 150 2-30, av.15.

et er, 12.6 10 Ethyl acetate, 200 do NazSOi, 8 150 2-15, av. 8. 12.6 15do do Ethyl centralite. 0.3 g.; NaiSOi, 8 150 2-25, av. 10.

Polyethleneglycol, 0.5 g. 12.6 20 Methyl ethyl ketone, 170; Methylcellulose, 1,000 Ethyl centralite, 0.3 g.; NaCl, 4. 150 2-40.

Methanol, 30. cps., 2.1. Turkey red 011, 1.6 g. 13.43 20 Methyl ethylketone, 170; o do NaCI,4 150 1-10.

Acetone, 30. 12.6 20 do do rln 150 1-20. 13.35... 10 Methyl acetate, 200Methylo cellulose, 1,500 Ethyl centralite, 0.3 g 150 1.

cps., Ethyl acetate, 200 Agar-agar, 4 do 150 -40. do Agar-agar, R 1605-15.

Agar-agar, 12 do 150 15-30. Polyvinyl alcohol, 2 .-d0 200 1-2. Polyvinylalcohol, 4 do 200 Gelatin, 9 do 150 5-10. Gelatin, 6 do 150 5-30, av.10-15 Gelatin, 12 do 150 l-40. Gelatin, 4 do 150 5-30. Gelatin, 8 do 1505-15. do do 150 5-15. 10 0 do do 150 l-l5. N itrometliane, 900 Gelatin,'12 do 150 5-30. 10 Ethyl acetate, 200 Gelatin, 8; Alginate, 0.5--Diphenyl aimne, 0.2 g 150 2-10. 10 (10 Alginate, 6 do 150 7-15. 10Alginate, 10 Ethyl centralite, 0.2 g-- 150 5-20. 10 Algiiiate, 12 1505-15. 10 Polysodium acrylate, 3.-.. 150 20-40. 10 Polyvinyl methylether- 160 l-lO.

malcic anhydride, 4. 20 Nitromethane, 200 Polyvinyl methyl ether----..d0 NazS0i,12.. 150 2-20.

malcic anhydridc, 6.

1 Kelcoloid LV.

Although this invention has been described with refer- References Citedin the file of this patent ence to illustrative embodiments thereof, itwill be appar- UNITED STATES PATENTS ant to those skilled in the artthat it may be embodied in 2,027,114 Olsen et 1 J 7 19 other forms butwithm the scope of the appended claims. 2,160,626 Schaefer May 30, 1939W l i 2,213,255 Olsen et al. Sept. 3, 1940 1. Cellulose nitrate,containing more than 12% nitro- 2375175 Slug May 1945 L 2,715,574 CoxAug. 16, 1955 gen, in the form of substantially spherical, non-porousparticles having a maximum average diameter of about 50 microns, and amaximum diameter of about microns, and a minimum density of 1.4, saidparticles being OTHER REFERENCES Chemical Engineering, vol. 53, No. 12,December 1946, pages 92-96 (article by Olive).

1. CELLULOSE NITRATE, CONTAINING MORE THAN 12% NITROGEN, IN THE FORM OFSUBSTANTIALLY SPHERICAL, NON-POROUS PARTICLES HAVING A MAXIMUM AVERAGEDIAMETER OF ABOUT 50 MICRONS, AND A MAXIMUM DIAMETER OF ABOUT 100MICRONS, AND A MINIMUM DENSITY OF 1.4, SAID PARTICLES BEINGCHARACTERIZED BY THEIR ABILITY TO BE DISPERSED IN A PLASTICIZER TO FORMSTABLE, FLUID SUSPENSIONS.